Name Boundary lab Per Date The vast majority of earthquakes occur at plate boundaries. The deepest earthquakes occur at subduction boundaries where lithosphere is plunging down into the mantle. Deep focus earthquakes are defined as those with foci (plural of focus) occurring deeper than 300 kilometers. Shallow earthquakes have foci less than 70 kilometers deep, while intermediate focus earthquakes occur at depths between 70 and 300 kilometers. The behavior of the subducting plate is determined by the age of the rocks comprising the plate. Older crust is cooler, and therefore denser, than younger crust. Older, cooler, denser crust subducts faster and at a steeper angle than younger, warmer, less dense crust. In this lab activity, you will plot and compare actual earthquake foci data from two areas where subduction is currently occurring. One of the areas includes the Tonga Islands in the southwest Pacific Ocean. The other area is along the coastline of Chile. Procedure: 1. Look at Earthquake Depth Data Table (On next page). Determine the number of shallow (<70), intermediate (70-300) and deep (>300) earthquakes and record them all on the summary table, for both the Tonga and Chile Trench. 2. Add the numbers in both columns of Summary Table. The total should equal the Total number of Quakes. Summary Tablle Earthquake Type Focus Depth Tonga Chile Shallow Intermediate Deep Less than 70 km 70 km 300 km More than 300 km Total number of Earthquakes - 1 -
3. On the graph below plot the data for Tonga trench from the table. DO NOT CONNECT THE DOTS. Instead draw a Best fit line for the points. A Bestfit line is a line that does not go through the points but shows a trend of data. Assume the line is the upper surface of a subducting plate. Label the approximate location of the Tonga trench on the graph indicate the name of the two converging plate indicate the relative movement of each plate using arrows indicate the type of crust that makes up each plate. 4. Make a 2 nd graph for the Chile trench. Plot the Chile data and again draw your best-fit line. Assume the line is the upper surface of a subducting plate. Label the approximate location of the Chile trench on the graph indicate the name of the two converging plate indicate the relative movement of each plate using arrows indicate the type of crust that makes up each plate. Earthquake Depth Data Tonga trench Chile Trench Longitude ( W) 176.2 175.8 174.9 175.7 175.9 175.4 174.7 176.0 175.7 173.9 174.9 178.5 177.9 179.2 178.7 178.3 177.0 174.6 178.8 176.8 177.4 178.0 174.1 179.2 178.8 178.1 175.1 178.2 176.0 178.6 174.8 178.2 179.1 177.8 177.0 Focus depth (km) 270 115 2 190 2 1 205 580 5 565 6 0 5 0 580 3 420 520 5 30 465 670 590 510 5 220 615 595 675 4 380 Longitude ( W) 67.5 66.9 68.3 69.3 62.3 70.8 61.7 68.4 69.8 66.5 69.8 67.3 67.7 69.5 68.3 67.9 69.1 69.2 63.8 68.6 66.7 68.1 66.7 65.2 67.5 69.7 68.2 67.1 66.2 66.3 68.6 66.4 68.5 65.5 68.1 Focus depth (km) 180 175 130 480 5 120 30 220 55 185 120 75 110 1 95 345 125 210 145 200 285 170 1 230 230 215 180 2 1 290 130 Boundary Lab 2 HGB 12/16/1999
Tonga Trench 180 179 178 177 176 175 174 173 172 171 170 700 0 0 0 300 200 100 0 Chile trench 70 69 68 67 66 65 64 63 62 61 0 0 Depth in Km 300 200 100 0 Depth in Km Boundary Lab 3 HGB 12/16/1999
Analysis and Conclusion 1. Using the reference tables, identify the following as a convergent, divergent or transform boundary: a. Mid Atlantic ridge b. Tonga trench c. Aleutian trench d. East Pacific Rise e. Great Rift Valley f. San Andres Fault 2. Which tectonic plate is being subducted at the Tonga trench? And at the Chile Trench? 3. Compare the distances of the Tonga and Chile trench from the East Pacific Rise, which is closer and which is farther? If the East pacific rise is the source of new rock, which trench would contain the oldest rock? Why? 4. Look at the summary data compare the depth of the majority of the earthquakes at the Tonga trench and the Chile trench. 5. Collision of which two types of plates create the deepest Quakes? 6. Which type of plate boundary creates new ocean crust? Boundary Lab 4 HGB 12/16/1999
7. From looking at your reference tables where do we usually find divergent boundries? 8. Which Geographic feature is is associated with subduction zones? 9. What is the relative movement of the plate along the San Andreas Fault? Credits: Namowitz and Spaulding. Earth Science. Boston: McDougal Littell, 1994. Osmun, Vorwald & Wegner. Explorations in Earth Science. Albany: United Publishing Co., 1995 Boundary Lab 5 HGB 12/16/1999